High-Speed Packet Access (HSPA) provides a major extension of the Wideband Code Division Multiple Access (WCDMA) radio interface. With HSPA it is now possible to provide mobile broadband using High-Speed Downlink Packet Access (HSDPA) and High-Speed Packet Uplink Access (HSUPA). Thus, HSPA can be seen as a complement and replacement to other types of broad band access such as Asymmetric Digital Subscriber Line (ADSL).
In WCDMA, cells may become overloaded. The overload may be due to several reasons and may result in different problems, such as too high downlink (DL) transmit power, starvation in an uplink (UL) scheduler or too high UL rise-over-thermal (RoT). The RoT indicates the ratio between the total power received from wireless sources at a base station and the thermal noise. The RoT is typically used as a measure of how congested a cellular network is. The RoT is especially important to keep under control, since if it becomes too high the system may become unstable and very high RoT values may occur. Therefore the capacity of the cellular network is often determined by how high the acceptable RoT is. A too high RoT decreases the coverage and may lead to dropping of some services, especially for users close to the cell border. To avoid this, there is typically a congestion control (CC) function that takes actions in a controlled manner if the RoT becomes too high. The congestion control function may take action to lower the rate for user equipments (UEs) in a CELL_DCH state or even drop users if necessary.
The dominant mobile broad band service today is down-link dominated, using an interactive bearer such as HSDPA and HSUPA, also known as Enhanced Uplink (EUL). Services such as video-sharing via a website, e.g. YouTube, and web browsing are examples of services that give rise to down-link heavy traffic. Even though these services are down-link heavy, the uplink in WCDMA still creates a lot of interference and contributes to raising the RoT. The reasons for this are the continuous control signalling in UL and the TCP HTTP (Transmission Control Protocol Hypertext Transfer Protocol) request and ACK/NACKs (acknowledgements/negative acknowledgements) in the UL. This means that the UL must reside in an active state since there is either DL transmission to receive or occasional UL data to transmit, and this creates interference and increases the RoT.
When congestion occurs in a cell due to too high UL RoT, typically the only option left is to release a random interactive bearer. This gives a negative impact on the user quality of service (QoS) which it is of interest to try to avoid.